1. Field of the Invention
The present invention relates to a thin flat cable configured to transmit radio frequency signals, and an electronic device including the flat cable.
2. Description of Related Art
Conventionally, a coaxial cable exists as a typical example of radio frequency line for transmitting radio frequency signals. A coaxial cable includes a central conductor (signal conductor) shaped to extend in one direction (shaped to extend in the direction of signal transmission), and a shield conductor provided concentrically along the outer peripheral surface of the central conductor.
Incidentally, as radio frequency devices including mobile communications terminals have become increasingly smaller and thinner in recent years, it is not possible to secure a space for disposing a coaxial cable inside the terminal housing in some cases.
Accordingly, attention is being given to use of a flat cable as discussed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 for such a terminal housing. Although a flat cable has a larger width than a coaxial cable, a flat cable can be reduced in thickness, which proves particularly advantageous for cases such as when there is only a small gap inside the terminal housing.
The flat cable discussed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 has a tri-plate strip line structure as its basic structure.
The flat cable as discussed in each of International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 has a flat plate-like dielectric element having flexibility and an insulating property. The dielectric element has an elongated shape extending in a straight line. A second ground conductor is provided on a second surface orthogonal to the thickness direction of the dielectric element. The second ground conductor is a so-called solid conductor pattern that covers substantially the entire second surface of the dielectric element. A first conductor is provided on a first surface opposite to the second surface of the dielectric element. The first ground conductor includes elongated conductors, which are shaped to extend along the longitudinal direction, at both ends of the width direction that is orthogonal to the longitudinal direction and the thickness direction. The two elongated conductors are connected by bridge conductors. The bridge conductors are disposed at predetermined spacings along the longitudinal direction, and shaped to extend in the width direction. Consequently, the second ground conductor has a shape such that openings with a predetermined length are arranged along the longitudinal direction. The bridge conductors for forming these openings are generally disposed at regular spacings along the longitudinal direction.
A signal conductor with a predetermined width and a predetermined thickness is formed in the middle of the thickness direction of the dielectric element. The signal conductor has an elongated shape that extends in a direction parallel to the elongated conductors of the first ground conductor and the second ground conductor. The signal conductor is formed substantially at the center of the width direction of the dielectric element.
With the above-mentioned configuration, in plan view of the flat cable (when viewed in a direction orthogonal to the first surface and the second surface), the signal conductor is disposed so as to overlap the first ground conductor only in the location of the bridge conductors, and lie within the openings in other areas.
The flat cable as discussed in International Publication No. WO 2011/007660 and Japanese Registered Utility Model No. 3173143 includes interlayer connection conductors in order to provide electrical continuity between the first ground conductor and the second ground conductor. The interlayer connection conductors penetrate the dielectric element in the thickness direction. The interlayer connection conductors are formed in the connecting areas of the elongated conductors of the first ground conductor with the bridge conductors.
However, further reductions in width are desired for the thin flat cable as mentioned above. In order to ensure that a flat cable has the same characteristic impedance after being reduced in width as that before the reduction in width, it is necessary to narrow the width of the elongated conductors of the first ground conductor.
However, in the case of the structure in which the interlayer connection conductors formed by conductive vias are formed in the elongated conductors as mentioned above, the narrower width of the elongated conductors means that the diameter of the interlayer connection conductors also needs to be made smaller. The interlayer connection conductors are each formed by filling a through-hole or excavated hole with a conductive paste and then curing the conductive paste. Consequently, when the interlayer connection conductors have a small diameter, the interlayer connection conductors are susceptible to breakage from the stress exerted when the flat cable is bent. Moreover, the narrow width of the elongated conductors makes it difficult to form the through-hole or excavated hole at a predetermined position, leading to an increase in work load or a decrease in manufacturing yield. Further, in a case where the bridge conductors are narrow, the stress exerted on the interlayer connection conductors when the flat cable is bent is exerted on the bridge conductors located near the interlayer connection conductors having high hardness, which may cause the bridge conductors to break in some cases. As described above, with the conventional structure, narrowing the width of the flat cable leads to a decrease in the reliability of the flat cable.